1. Field of the Invention
The invention relates to a method of making specimens for examination by means of a particle-optical microscope apparatus.
The invention also relates to a device for carrying out the method and to a particle-optical microscope apparatus comprising such a device.
2. Related Art
Many materials of interest nowadays from the point of view of materials science have properties which are determined by their microstructure; examples in this respect are materials with crystallites of dimensions in the nanometer range, multilayer structures such as used for X-ray reflection, and microelectronic components. In order to understand their macroscopic properties, it is of essential importance to determine the structure properties at an atomic scale. Also examinations at a molecular scale are important not only for materials science also for biological specimens as well.
The properties at such a small scale can be determined by means of a particle-optical microscope apparatus such as an electron microscope. Therefore, it is necessary to make specimens for examination in such apparatus from which the desired material properties can be determined. Such specimens should preferably be so thin that they are transparent to electrons of an energy which is customary in electron microscopes.
For electron microscopy there are a number of methods of making electron-transparent specimens which are known, for example pulverizing, ultramicrotomy, chemical polishing and ion milling. Each of these methods, however, has its own drawbacks, which make the relevant method unattractive or even completely unsuitable for making specimens for the purpose of materials examination at an atomic scale.
Upon pulverization of multi-component systems (such as, for example an integrated semiconductor in which metal connections are attached to semiconductor material), the boundaries between the components are usually lost. Particularly these boundaries are of interest to materials science, so that specimens made according to this method can be used to a limited extent only. Ultramicrotomy is a method of making slices of specimens with a thickness of more than 5 nm. This method, however, cannot be used very well in the case of hard materials because the large forces occurring during cutting cause many faults (such as dislocations and fractures) in the material to be studied, so that an incorrect impression of the structure of the material is obtained. For chemical polishing a liquid with a solvent is conducted across the specimen material (for example, acid across metal). If the specimen is thin enough, the supply of liquid is terminated. After this treatment, however, a surface layer remains which has a structure and/or composition which deviates from those of the original material. The structure which remains often has the appearance of an amorphous layer. Ion milling is a technique for the formation of specimens by subjecting a thin slice of the specimen material to a surface treatment by charged particles, so that surface layers of the slice of the specimen material are removed. The charged particles (ions) are applied to the specimen with an acceleration voltage of a few kilovolts at a small angle (for example, 10.degree.) relative to the specimen surface. This technique enables the manufacture of specimens which are sufficiently thin for the described materials examination. However, this method of surface treatment also appears to leave an amorphous layer behind on the specimen surface.
An amorphous layer on the surface of a specimen, or an amorphous region which partly covers the surface, seriously hampers the described examination. An amorphous layer which completely covers the specimen leads to blurting of the image of the specimen; moreover, the chemical analysis of the surface thus becomes less reliable. It has been found that a partly covering amorphous layer acts as a kernel for the growth of a fully amorphous layer when the specimen is exposed to an electron beam, so that the already described problems relating to a completely amorphous surface occur.